1. Field of the Invention
The present invention relates to a micro thin-film structure, a MEMS (Micro Electro-Mechanical System) switch employing such a micro thin-film structure, and methods of fabricating the micro thin-film structure and the MEMS switch, and in particular to a micro thin-film structure, which is improved in lamination structure to minimize the deformation of the micro thin-film structure and allows a MEMS switch to be stably operated when the micro thin-film structure is applied to a movable electrode of the MEMS switch, a MEMS switch employing such a micro thin-film structure, and methods of fabricating them.
2.Description of the Related Art
Among RF devices fabricated using MEMS techniques, switches are most widely manufactured at present. RF switches are frequently applied to circuits for signal selection and transmission or impedance matching in radio frequency communication terminals and systems of microwave band or millimeter wave band.
An example of such an RF switch is disclosed in Japanese Patent Publication No. Hei 10-334778 issued on Dec. 12, 1998 and entitled “Critical Microswitch and Its Manufacture.”
Briefly, the microswitch comprises a movable electrode initially deformed by difference in residual stress, a fixed electrode spaced from the movable electrode, a movable electrode support portion for supporting both ends of the movable electrode, and a fixed electrode support portion for supporting the fixed electrode.
FIG. 1 is a perspective view showing a construction of a conventional MEMS switch, and FIG. 2 is a cross-sectional view taken along line I-I′ of FIG. 1.
Referring to FIGS. 1 and 2, a signal line 3 having a dome-shaped contact 3a is formed on a substrate 2 at the central part of the top side of the substrate 2. A movable electrode 6 is positioned above the dome-shaped contact 3a, wherein the movable electrode 6 is fixed in a form of a simply-supported beam by spacers 4. A through-hole 3b is formed through the top of the dome-shaped contact 3a. A pair of fixed electrodes 7 are respectively positioned on the opposite sides of the signal line 3, wherein the fixed electrodes 7 cooperate with the movable electrode 6 to generate electrostatic force, thereby drawing the movable electrode 6 to come into contact with the dome-shaped contact 3a. The movable electrode 6 has a double thin-film structure having an electrode layer 6a formed from a conductive material and a reinforcement layer 6b formed on the top side of the electrode layer 6a to reinforce the strength of the electrode layer 6a. 
In such a conventional MEMS switch, electrification is produced between the fixed electrodes when DC voltage is applied to the fixed electrodes 7 and the movable electrode 6 is drawn toward the substrate 2. As the movable electrode 6 is drawn, the central part of the movable electrode 6 comes into contact with the dome-shaped contact 3a. 
In order to ensure the stable switching operation of such an MEMS switch, it is necessary for the movable electrode 6 to maintain a horizontal posture without being deformed. However, there is a problem in that because the length L of the movable electrode 6 is relatively very large as compared to the distance d between the movable electrode 6 and the substrate 2, the movable electrode 6 is easily bent. Accordingly, a structure is demanded for effectively improving the flexural strength of the movable electrode 6.
However, the interface of the electrode layer 6a and the reinforcement layer 6b of the conventional movable electrode 6 is formed only as a horizontal plane A. Therefore, if stress is generated due to a difference in residual stress or thermal expansion coefficient caused in the electrode layer 6a and the reinforcement layer 6b after a thin-film has been formed, a face for canceling the generated stress is formed only by a horizontal plane. Therefore, there is a problem in that the effect of preventing the deformation of the movable electrode is insufficient.
Such deformation of a thin film structure may cause a problem not only in the above-mentioned MEMS switch but also in other devices employing MEMS techniques.